[225Ac]Macropa, a Match Made in Ithaca

Introduction: Actinium-225 is an ideal candidate for targeted α therapy (TAT) but the development of ²²⁵Ac-based radiopharmaceuticals presents challenges due to a poor understanding of its coordination chemistry, a lack of ²²⁵Ac-specific chelators, and the unknown long-term stability of ²²⁵Ac labeled drug products. To date only two macrocycles, DOTA and Macropa, have been used clinically to form stable ²²⁵Ac complexes for TAT. This work comparatively examines labeling conditions and stability of selected Macropa- and DOTA-containing PSMA targeting drugs.

Methods: A bifunctional version of Macropa (M-NH₂), four Macropa-containing targeting vectors, and an equivalent DOTA-containing targeting vector were radiolabeled with ²²⁵Ac in 1 M ammonium acetate to yield clinically relevant concentrations (25-35 μCi/mL) of labeled product. Labeling conditions, radiochemical stability, and the effect of metal impurities on radiolabeling yields were investigated using thin-layer chromatography (TLC) as a quality control method.

Results: [²²⁵Ac]Macropa labeling experiments conducted at 25°C consistently showed ≥ 99% radiochemical yields using Macropa concentrations as low as 1-10 μM (approximately 50:1 L:Ac molar ratio). No ²²⁵Ac release was observed over a seven-day period for all [²²⁵Ac]Macropa complexes studied. Comparatively, incubation of an analogue DOTA-containing compound ²²⁵Ac at room temperature did not produce a measurable isotope incorporation, while showing quantitative labeling at 90°C. Also, formation of [²²⁵Ac]Macropa was unaffected by the presence of common metal contaminants introduced in the labeling solution (50 ppm) at 25°C. In contrast, the formation of the [²²⁵Ac]DOTA complex at 95°C was dramatically impacted (54.4% < RCP < 78.7%), even when metal contaminants were present at a 10-fold lower concentration (5 ppm). Furthermore, ²²⁵Ac quantitatively displaced pre-chelated Ce³⁺, Bi³⁺, Sm³⁺, and Lu³⁺ ions from Macropa within 30 minutes at 25°C or 95°C (Ce³⁺ only). Macropa was also found to readily recapture ²¹³Bi (92.5 ± 1.2%) resulting from ²²⁵Ac decay.

Conclusions: Macropa displays exceptional selectivity for ²²⁵Ac, producing highly pure and stable drug products. Quantitative labeling is achieved at 25°C, regardless of the presence of impurities, which makes Macropa amenable to more structurally sensitive targeting vectors (e.g., proteins and antibodies). The ability of Macropa to readily re-complex ²¹³Bi in the drug product formulation is highly attractive as it will help minimize any nonspecific radiotoxic effects in vivo. Collectively, these results establish Macropa as the best-in-class ²²⁵Ac chelator for TAT products.